Effect of surface hydrophobicity on short-range hydrophobic attraction between silanated silica surfaces

The interaction forces between silanated silica surfaces without nanobubbles were measured using colloidal probe atomic force microscopy (AFM). To obtain hydrophobic surfaces without nanobubbles, an aqueous solution was introduced between the surfaces following an exchange process involving several solvents. In the obtained approaching force curves, an attractive force was observed from a distance of 10-20 nm, which is an additional attractive force stronger than typical van der Waals attractions. When the surface hydrophobicity decreased, the range of this attraction decreased slightly; the attraction disappeared when the surface contact angle was below 90°. In contrast, measurements in the water-ethanol mixtures revealed that the attraction persisted even when the contact angle was well below 90°. The possible origin of this force was discussed on the basis of the obtained results.